Delignification: breaking down the chemical structure of lignin

Kappa number: measure of the lignin content in the pulp

Pulp viscosity: measure of the average chain length of cellulose (DP- degree of polymerization)

Fiber Liberation Point: (130-180oC) – occurs when sufficient lignin has been removed during pulping- chips are broken apart into fibers with little or no mechanical action.

Full chemical pulp: produced by chemical methods only; yield about 50%, lignin content 3-5 %

Unbleached pulp: full chemical pulp as it comes from the pulping process, light to dark brown in color

Bleached pulp: white pulp produced by bleaching full chemical pulp

Dissolving pulp: bleached chemical pulp; yield 30-35%, high cellulose content (95% or higher)
Kraft process with acid prehydrolysis or acid sulfite process; Improved cellulose purity achieved by a cold alkali extraction. Used for cellulose derivatives production (rayon, Cellulose acetate, cellophane).

Kappa number: consumption of 0.1 N KmnO4 in milliliters consumed by one gram of pulp in 0.1 N sulfuric acid (H2SO4) used to monitor the degree of delignification of chemical pulps after pulping and between bleaching stages.

Klason lignin, acid soluble lignin: gravimetric, direct method; KL- residue after total acid hydrolysis of carbohydrates portion of wood/pulp. Acid soluble lignin can be estimated spectrophotometrically (UV region). For hardwood pulps: Klason lignin, % = 0.15 x Kappa number


Full chemical pulping with NaOH and Na2S, pH above 12 , 160-180 oC , 800 kPa (120 psi), 0.5-3 hrs
Useful for any species of wood, high strength of pulp, tolerant to bark, efficient energy and chemicals recovery cycle
Disadvantages: difficult to bleach compare to sulfite pulps, low yields (carbohydrate losses), sulfur compounds- extremely offensive odor 80% of lignin dissolved, 50% hemi, 10 % of cellulose

Soft cooks- (for bleached grades) lignin content 3.0-5 .2 % (20-35 Kappa #)
For softwoods, or 1.8-2.4 % lignin (12-18 Kappa #) hardwoods
Medium soft cooks- (bag papers) lignin- 5.2-7.5% (35-50 Kappa #)
Hard softwood cooks- 9-11 % lignin (65-70 Kappa #)- top liner boards 12-16.5 % lignin  (80-110 Kappa #) – bottom liner

Pressurized vessel for cooking chips into pulp


Mill has a bank of 6-8 digesters

Heating:  Direct- with the steam, dilutes the cooking liquor, changes the liquor to wood ratio, bad uniformity of cooking-disadvantage Indirect- cooking liquor heated outside the digester, heat exchanger. More uniform heating and cooking.

Cooking time: time from initial steaming of chips to start of digester blowing

Time to temperature: Time from initial steaming of chips to the point when desired temperature is reached

Time at temperature: Time from when the cooking T is reached until digester blow starts

Cooking sequence:

- Filling of the digester with chips, white liquor, and weak black liquor
- After initial circulation of the liquor (contents settle) additional chips are added
- Digester sealed and heating starts; temperature rises for about 90 min until the cooking temperature is achieved.
- Cooking T is reached- cook at the temperature 0.5-3 hrs
- During the heating time, air and other gases from the digester are vented
- When cook is completed (Kappa #), content of the digester is discharged to the blow tank
- Digester is opened and the sequence is repeated

B. Continuous Digester

Tube shaped vessel, with elements: presteaming, liquor impregnation, heating, cooking, and washing Chips enter and exit the digester continuously More space efficient, easier to control, labor saving, energy efficient than batch digester
Rotary valves help to fill the digester, from atmospheric to high pressure

Vertical digester (One to two vessels) 77 million tons/year
3 zones: heating, cooking, washing

M&D Digester (Messig & Durkee)
Inclined at 45 degree
Kraft pulping of sawdust, acid prehydrolysis (with kraft), and semichemical pulping
Cooking time approx. 30 min
Diameter about 2.4 m- relatively small production levels

Horizontal digester, multi- tube (2-8 digester)
Screw feed
Kraft pulping of sawdust
Semichemical pupling of chips, non-woods (straw), short cooking times

Digester blowing
Continuous digester- @ about 100oC
Batch digester- @ or near cooking temperature 170 oC, pulp loses a significant amount of strength (10-15%)
Vapor expansion in chips is usually sufficient force to cause fiber separation

Blow tank
Large cylindrical vessel
Hot pulp from digester is mixed with agitators with diluted black liquor

Hot gases from BT are recovered by the blow heat accumulator (heat exchanger). Condensates sometimes used as dilution water, in brown stock washing, etc. Non-condensable gases often diverted to the lime kiln for combustion

Liquor to wood ratio
3:1 to 4:1 in full chemical pulping
It is kept as small as possible while maintaining good digester operation including good circulation for even cooking

Rejects portion (knots) not sufficiently delignified to liberate Knotter- coarse screening equipment (3/8 in holes)
Rejects in kraft pulping are repulped. After washing, pulps screened to remove shivers, dirt, contaminants. Shorter fibers (hardwoods) screen more easily than softwoods. Pulp screened prior to washing for improved washing efficiency

Rotary vacuum washers
Drum washers with wire-mesh covered cylinder
Drop leg of pipe supplies the washer with a vacuum
As the drum contacts the slurry, a vacuum is applied to thicken the stock
Typical configuration: 3-4 washers
80 % of the contaminants are removed at each stage
Double wire press
Suitable for market pulp production
Brown stock washing, washing between bleaching stages, or stock thickening
Black Clowson
Continuous, wire countercurrent washer

A- Evaporation- concentration of black liquor
Multiple- effect evaporators (7 pcs)
15% solids in dilute black liquor (from 1 washing stage) to 65-70% solids after concentrator
Steam- black liquor countercurrent flow

B- Combustion- recovery boiler
Burns the concentrated black liquor by spraying it into the furnace through side openings
The water evaporates and the organic materials removed from the wood form the char and then burn
Fuel value of black liquor (65-70% solids) is 14-16 MJ/kg compare to coal 32 MJ/kg
Overall chemical reactions in the recovery boiler in addition to combustion are:
2 NaOH + CO2 = Na2CO3  +H2O
(Conversion of sodium salts)

Na2SO4 + 4 C = Na2S + 4 CO
(Reduction of make-up chemicals)

Firing –up black liquor at 65% solids leads to a maximum combustion temperature of 1100-1300oC (2000-2400 oF)

The recovery boiler is the largest, single most expensive piece of equipment in a kraft mill costing over $100 million
Molten slag leaves the boiler directly to the green liquor dissolving tank
Causticizing process
Green liquor clarifier
Slaker CaO + H2O => Ca(OH)2
Na2CO3 + Ca(OH)2  => 2 NaOH + CaCO3
White liquor clarifier - digester
Lime kiln, 3 zones: Drying, heating, calcining

CaCO3  => CaO + CO2 (g)
Causticizing efficiency 78-80 %

Causticizing efficiency = NaOH/ [NaOH + Na2CO3 ] x 100 [%]

Cooking: (Pulping liquor) Black liquor + White liquor + wood moisture
Black liquor (residual alkali, Na2S, NaOH)
White liquor (conc. Na2S, NaOH)
Wood moisture  H2O

Black Liquor: (spent liquor)
Residual alkali, Na2S, NaOH

Green liquor (after combustion)
Na2S, Na2CO3

White liquor- fresh pulping liquor
NaOH+ Na2S, (Impurities, salt, Na2CO3, water, corrosion, etc.)